This invention concerns a three-phase direct-arc electric furnace fed with a controlled current and also a method to feed with controlled current a three-phase direct-arc furnace.
This invention is applied to three-phase electric arc furnaces for the smelting of metals and iron and alloys thereof in particular.
Direct-arc electric furnaces are mainly used at the present time for the smelting and re-smelting of steel and are almost all three-phase furnaces.
During the last twenty years the power of each furnace has increased considerably, passing from unit powers of 16 MW and 20 MVA to powers greater than 85 MW and 120 MVA.
These high powers entail for the supply network great problems of disturbances in the voltage (flicker) as well as considerable phase shifts due to the inductive loads.
To correct the phase differences due to these inductive loads and reduce the voltage fluctuations, the modern compensation technology makes use of variable compensators of reactive power, which are operated with controlled diodes.
The principle of regulation is shown in FIG. 1 and is as follows.
Three inductors are placed in parallel connection with the three-phase medium-voltage line, which is the supply point of the strongly inductive loads of the furnace; these inductors are supplied by means of thyristors T, the firing angle of which is controlled on the basis of the current detected by the device S1.
This regulation system keeps constant and balanced at zero the total reactive power employed by the furnace, the inductors L1 and L2 and the batteries of power-factor corrective capacitors CR, which are all connected to the medium voltage supply line.
The batteries of power-factor corrective capacitors CR with the addition of suitable inductors are made to perform also the function of filtration of the harmonics generated by the furnace and by the compensation system.
Instead, the active power of the furnace arcs is regulated by changing the height of the electrodes by means of suitable hydraulic assemblies GI, by trying to keep the resistances of the arc constant.
To overcome the difficulties and some of the shortcomings which this indirect type of regulation of the absorbed current entails, direct-current furnaces have recently been produced, and with this type there is one single electrode and the return of the current is through the shell of the furnace.
The supply current of the arc is provided by a rectification assembly made of controlled diodes or thyristors. This system involves two substantial drawbacks. On the one hand there is the difficulty of obtaining the return route of the current, while on the other hand there is a strong generation of harmonics of an odd number by the rectification system.
To obviate these serious drawbacks of both types of arc furnace, the present applicant has designed, tested and embodied the present invention, which has as its objective to overcome the problems of the prior art.